Practiced at present in the world is the well-known technique of enamelling in kilns which is being constantly improved due to the development of new modern equipment. Yet, the use of firing kilns is a big obstacle in the way of developing the art of enamelling on an industrial basis which contracts the scope of employing vitreous enamelling as a means of protecting metal articles in various branches of the industry. The point is that firing kilns fail to offer the opportunity of observing the process of fusing the enamelling slip and controlling said process with the result that the finely-ground slip is exposed to excessively high destructive temperatures. Moreover, the use of firing kilns requires additional self-contained equipment to cope with the application of enamelling slips, drying of coating, straightening and cooling of the tubes which occupies extra floor area.
Modern firing kilns are provided with powerful telescopic hoists, big mechanical, hydraulic and electrical facilities which serve to maintain high process temperatures in the kiln. This invites operating difficulties and renders the apparatus a sophisticated piece of machinery. The enamel applied to tubing must meet high strength requirements and display a uniform thickness everywhere. But in a firing kiln tubes are subject to bending and this defect impairs the quility of the metal coating applied. To eliminate bends, use is made of straightening machines and this renders the enamelling plant as a whole even a more complicated machine.
The disadvantages mentioned above can be eliminated by a recourse to the induction heating process of tube enamelling which enables the continuous technique of applying an enamel coating either to the inside or outside surface of the tubes or to both surfaces simultaneously.
Under this method, all the operations of applying the enamelling slip, drying, fusing and cooling are carried out successively and simultaneously. The capacity of the induction enamelling process is 5 to 8 times that of the kiln process, and there is the possibility of eliminating local defects without stripping the entire coating when the induction process is employed. In the case of the kiln process, defects can be rectified by re-enamelling of the entire tube surface starting with the stripping of the enamel from the metal surface and finishing with the multiple cycles of slip application, drying and fusing the coating or with the patching of the spoiled placed by fillings in platinum or gold.
There is known in the art an apparatus for the induction process of tube enamelling (cf., for example, Bundesrepublic Deutchland Patentamt No. 2,503,232) comprising an upender with grippers which serves to lift the tubes from a feeding conveyor and transfer them from the horizontal position into a vertical one, a means of applying enamelling slip to at least one side of each tube leaving the upender, a heating means designed to dry and fuse the enamelling slip and a means of hauling the tubes through all stages of the process in succession.
Employed as the means of hauling there are two rotors installed vertically next to each other and provided with grippers for holding the tubes in an upright position during the hauling and processing, one of the rotors catering for the loading and unloading of the tubes and the other displacing the tubes from work station to work station. For transferring the tubes from one rotor to another, use is made of a transfer mechanism. Running contiguously with the loading/unloading rotor is a conveyor for the horizontal conveying of the tubes which has a built-in upender placing the tubes into the vertical position when they are transferred to the loading/unloading rotor and returning them back into the horizontal position when the tubes are removed from this rotor. Each of the tubes is provided with a removable cap applied to one of its ends by means of which the tube is fixed to the loading/unloading rotor and to a carriage of the working rotor displacing along vertical guides.
In the known apparatus, difficulties are encountered during the operations of applying the enamelling slip in two layers, the prime coat and the top coat, at two work stations as well as during the drying and fusing of the slip due to the fact that the vertical position of the tubes is disturbed in the course of their displacement integrally with the rotors and along the vertical guides from station to station. Furthermore, any stopping of the rotors causes the tubes to swing with the result that their centering in the zones of applying, drying and fusing the slip is also disturbed. This results in defective enamel coating, reduced capacity of the apparatus and impaired safety of the personnel. The presence of two work station for the application of the enamelling slips complicates the construction of the apparatus and extends the way travelled by each tube in the course of enamelling.
The use of two vertical rotors installed next to each other also complicates the construction of the apparatus, calls for the necessity of transferring the tubes from one rotor to the other whereas the fact that the guides with the carriages are located on the working rotor adds to the weight of the rotating parts and extends the way the tubes are bound to cover in the course of treatment. This all impairs the operational reliability of the apparatus as a whole let alone such obvious thing that the conveyor and upender of the known design are capable of transferring tubes of only a certain length and diameter from the horizontal position into the vertical one.
Fork-shaped grippers provided on the rotors fail to cater for the enamelling of tubes of various diameter and they also cannot assure a reliable functioning of the upender in removing the tubes and hauling same by the rotors and along the guides in carriages with the result that the safety of the attending personnel can be endangered.